Which AV block is defined by progressively lengthening PR intervals with eventual dropped QRS, known as Mobitz I?

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Multiple Choice

Which AV block is defined by progressively lengthening PR intervals with eventual dropped QRS, known as Mobitz I?

Progressively lengthening PR intervals with an eventual dropped QRS complex is Mobitz type I, the Wenckebach pattern. This reflects a gradual delay in conduction through the AV node: each atrial impulse takes longer to traverse the nodal pathway, so the PR interval lengthens from beat to beat until one P wave is not followed by a QRS. After the dropped beat, the cycle repeats. The QRS is usually narrow because the block is at the AV node. This pattern can be influenced by autonomic tone or medications that slow nodal conduction, such as certain calcium channel blockers or beta-blockers. In contrast, Mobitz type II shows dropped beats without progressive PR lengthening—the PR interval remains fixed before a non-conducted atrial impulse. Third-degree (complete) block has no fixed relationship between P waves and QRS complexes, with independent atrial and ventricular activity. First-degree block simply elongates the PR interval but every P wave is followed by a QRS.

Progressively lengthening PR intervals with an eventual dropped QRS complex is Mobitz type I, the Wenckebach pattern. This reflects a gradual delay in conduction through the AV node: each atrial impulse takes longer to traverse the nodal pathway, so the PR interval lengthens from beat to beat until one P wave is not followed by a QRS. After the dropped beat, the cycle repeats. The QRS is usually narrow because the block is at the AV node. This pattern can be influenced by autonomic tone or medications that slow nodal conduction, such as certain calcium channel blockers or beta-blockers.

In contrast, Mobitz type II shows dropped beats without progressive PR lengthening—the PR interval remains fixed before a non-conducted atrial impulse. Third-degree (complete) block has no fixed relationship between P waves and QRS complexes, with independent atrial and ventricular activity. First-degree block simply elongates the PR interval but every P wave is followed by a QRS.

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